The purpose of this study is to expand an my previous research on physical play between adults and ADHD children. The study will focus on the effects of medication on adult-child physical play. The subjects will be 32 boys recruited from the population of children aged 5-9 who are referred by physicians for routine assessment for their response to medication at the Child Development Center of the University of California-Irvine. Children will participate in 5 five-minute standardized physical play sessions (four with a research assistant, one with the subject's father). These sessions will occur once each week during a 4 week randomized, double blind medication assessment (baseline, placebo, and 2 doses of Ritalin). The 4 sessions of physical play with a research assistant will vary depending on whether the child or adult is allowed to initiate the interaction, and whether the reaction of the adult is stimulating or unstimulating. In the ADULT-INITIATED STIMULATING (A-I:S) condition the adult will initiate highly stimulating physical play with the child. In the ADULT-INITIATED UNSTIMULATING (A-I:U) the adult will attempt to engage the child in quiet play. In the CHILD-INITIATED STIMULATING (C-I:S) the adult will respond positively only to requests to engage in active, physical play. In the CHILD-INITIATED UNSTIMULATING (C-I:U) condition the adult will respond positively only to requests by the child to engage in quiet games. The play session with the boy's father will be unstructured, except that physical play will be mentioned as a possible style of play. The data will be analyzed with a repeated measures MANOVA, repeated measure ANOVAS, event sequencing, and time-series analysis. It is generally expected that Ritalin will lower the levels of physical play, aggression, ratings of positive affect and negative affect, and approaching stimulation. However, the specific predictions depend on the extent to which the child is able to control his environment as well as the programmed reactions of the adult. The project has important health implications because it will shed light an the mechanism by which Ritalin affects behavior. Moreover, as described in the application, it will be an important step forward in basic research on temperament.